Dry sump theory

jaybird840 · 11-11-2010, 12:39 PM

Hi,

I'm having an issue with oil coming out my crankcase vent on my 914/6 conversion. The motor is a 2.7RS spec with webers (about an hour of total runtime so far on my rebuild). I installed a front-mounted tank. I'm not seeing a ton of oil, but probably 2-3 teaspoons per trip being puked out the crankcase breather--I'm positive that's where it's coming from. I am in the process of installing a puke tank to measure exactly how much, I'm currently just venting to the atmosphere via a chebby-style canister breather attached to a hose that extends about six inches above the vent. A trusted source (tons of race motor P-car experience) said that my front tank is likely too full.

This brings me to my question. If the oil pump is fed by gravity/suction, how does it know how much oil is above the inlet? I can see where the oil level above the inlet would produce a bit more pressure as the level increases, but can it be substantial? I can also see where overfilling the oil tank would cause the oil tank to puke from hot oil being returned from the scavenge side of the pump if you filled the tank without the car warm and idling, but not the crankcase.

I'd love to hear from the pros on this one... Thanks!

Elombard · 11-11-2010, 12:59 PM

Hmmmm crank case breather = presurized crank case = rings did not seat yet?

On a 911 CIS car for instance an over filled tank lets oil in the intake (oil fume recirc circuit) it does not cause the case to over fill...I dont think.

Come to think of it what keep the oil in the sump on a 911 from draining down into the case when the car is shut off. Possibly you have removed this part of the system. Or maybe its in the pump it self (check valve) where it pulls from the sump?

thinking out loud hopefully some one will chime in. Look for the diagram of the oil system I think its posted in tech section here; I believe its also shown in the Haynes manual.

Steve@Rennsport · 11-11-2010, 01:31 PM

JB,

I would do a leakdown on the engine to see if the rings have seated before doing anything with the oiling system.

T77911S · 11-11-2010, 02:09 PM

the question of "where does all the oil go" has been one of mine for a while.
my oil level gage is accurate to the dip stick. with oil topped off, when cold, my oil level goes in the red, once warm and idling, it is normal, but if accelerate, once again, into the red. my only conclusion is it has to be in the motor.

i seem to get more blow by (bad rings) with higher oil level and i go thru more oil. so i tend to leave it near min oil level.

agree- rings may not have seated.

i had a catch tank on my 914-6 conversion. it was the old washer bottle, tie rapped in engine bay and hose fit nicely. i just had to dump it every now and then.

dad911 · 11-11-2010, 02:27 PM

Quote:

Originally Posted by Steve@Rennsport

JB,

I would do a leakdown on the engine to see if the rings have seated before doing anything with the oiling system.

Yup. Spent a whole DE season chasing this issue on a 3.2 years ago. It ended up being a bad cylinder.

Grady Clay · 11-11-2010, 02:32 PM

Jaybird,

If I understand you correctly, you do not have the crankcase breather connected to the oil tank but connected to a “…chebby-style canister breather….”
If so, you have the system plumbed incorrectly.

With a front oil tank, the crankcase breather needs to go continuously up and over the cockpit and then down to the top of the oil tank. This must be one continuous vertical curve and not an up & down ‘S’ serpentine. It can’t trap any oil.

The point is that the engine must ‘breath’ gasses, vapors and oil from the crankcase breather to be recovered in the oil tank. The gasses and vapors are then vented to the atmosphere from the oil tank after any oil is separated.

Right now, you have a reasonably well sealed engine. You are only collecting a ‘small’ amount of oil in your separate catch tank. That will not always be the case. Even 2-3 teaspoons per trip will run the system out of oil before 500 miles.

You ask a good question about how the oil gets from the tank to the inlet side of the pressure pump.
Check this thread and links:
Oil Tank location

As you can see, I’m not a fan of the front-mounted oil tank.

I would like to see one of our vendors engineer a 997GT3-like engine mounted oil tank for the 964 and earlier engines.

Some of the features would be:
The oil tank close to the pressure pump inlet.
The hot oil is pressurized.
The oil cooler(s) and filter(s) in the pressure circuit.
A centrifugal separator in each circuit (pressure & scavenge) to eliminate entrained air.
A simpler dry sump oil system with less oil and less weight.

Best,
Grady

911pcars · 11-11-2010, 03:15 PM

As Grady advised. Took the words right out of my mouth. :-)

The crankcase breather receives a lot of high velocity oil vapor. The factory or remote oil tank helps reduce the velocity by having more volume than a hose or by add'l baffles in the tank. A separate breather (puke) tank can then vent the now-separated vapors to either the atmosphere or into the intake.

Sherwood